Pressurized-water nuclear reactors include a vessel, having a generally cylindrical barrel arranged with its axis vertical in the service position of the vessel and closed by a domed bottom head at its lower end.
The reactor core formed by juxtaposed fuel assemblies is placed inside the vessel in which, during the operation of the nuclear reactor, the reactor coolant flows, coming into contact with the core fuel assemblies in order to cool them and to transfer the heat produced by the core.
The nuclear reactor vessel contains internal equipment, called lower internals, for supporting and holding the core fuel assemblies inside the vessel.
These lower internals include, a cylindrical barrel arranged in a coaxial manner with respect to the vessel and a very thick core support plate integral with the lower end of the barrel.
The core barrel is suspended inside the vessel by its upper part and contains a baffle system fixed to the inner surface of the core barrel, making it possible to hold the fuel assemblies which rest on a lower core support plate by their lower part.
In order to prevent movements of the lower internals suspended by the core barrel inside the reactor vessel, devices are used for holding these internals making it possible to prevent, or to limit to a very low value, the movements of the lower part of the internals in radial and tangential directions.
In order to confront any accidental situation, devices are also used making it possible to retain the reactor lower internals, in the highly improbable situation in which a rupture of the core barrel occurs. The elements for retaining the internals may be made in the form of supporting means preventing the lower internals inside the vessel from dropping by an amount which would permit the centering pins for the core fuel assemblies to disengage from the centering and positioning orifices provided in the upper nozzles of the fuel assemblies.
Upper intervals arranged inside the vessel also include a lower part formed in the shape of an upper core plate into which are fixed fuel-assembly centering pins which have a part projecting beneath the upper core plate and which are engaged, during the installation of the upper internals inside the vessel, into openings made in the fuel-assembly nozzles. In this way, the fuel assemblies are so positioned, aligned and centered that a control rod assembly can be moved or lowered back down into them, in the event of a shutdown of the nuclear reactor.
The retention function of the lower internals in the event of rupture of the barrel is generally designated as a secondary supporting function, the internals and the core being supported, under normal conditions, by means of the core barrel which is suspended from the vessel, at the vessel flange onto which the vessel closure head is fixed.
In the case of the vessels of pressurized-water nuclear reactors which have been constructed according to the prior art and which are currently in operation, the two functions, namely the radial and tangential holding of the lower internals at the bottom part and the secondary supporting of these internals are provided by two entirely separate devices.
The lower internals are radially and tangentially held by keys which limit the rotational movements of the upper internals to a very small amount and which prevent the lower end of these internals from tilting.
The radial and tangential holding is achieved by means of six identical assemblies each comprising a key welded to the support bottom of the lower internals and a female part fixed onto the vessel and intended to receive, with a certain clearance, a part which projects towards the outside of the key. The female part itself comprises a solid piece welded to the internal wall of the vessel and keyhole slots for radially and tangentially holding the key which are fixed onto the solid piece by screws and positioning studs. The radial and tangential holding device allows the lower internals to move vertically inside the vessel and, in particular, allows the lower part of the internals to move vertically owing to expansions or contractions as the nuclear reactor is raised in temperature or is cooled.
The secondary supporting of the internals and of the reactor core, in the extremely improbable event of circumferential rupture of the core barrel, is provided by supporting columns fixed onto the lower face of the support bottom at one of their ends and onto the upper face of a fastener plate at their other ends, a base plate then bearing on the vessel bottom head and dampers fixed in an arrangement which is coaxial with respect to the supporting columns, these dampers being inserted between the lower fastener plate and the base plate.
The secondary support of the lower internals and of the core makes it possible to transfer and distribute, by means of the baseplate, the vertical forces on the vessel bottom head, to absorb the energy released by the drop of the lower internals and by the relaxation of the springs for holding the fuel assemblies in the event of rupture of the core barrel and to limit the vertical movement of the fuel assemblies in order to prevent the centering studs from disengaging, while at the same time, under normal operation, permitting movements which are due to differential expansion in the vertical direction between the lower internals and the vessel.
The radial holding devices according to the prior art, which include both keys fixed so as to project with respect to the peripheral part of the core support plate and keyhole slots fixed into a solid piece integral with the inner surface of the vessel are very bulky and disturb the flow of the cooling water inside the vessel.
The secondary supporting device for the internals is, moreover, relatively complicated.
Furthermore, during the installation on site of the nuclear reactor vessel, it is necessary to put the lower internals in place inside the vessel before the installation of the secondary support and of the radial holding keyhole slots.
A manhole is provided at the center of the support bottom in order to gain access to the vessel lower plenum, so as to measure the actual distance between the support bottom of the lower internals and the vessel bottom head as well as the relative positions in the circumferential and radial directions between the holding keys integral with the core support plate and the keyhole slot supports integral with the inner surface of the vessel.
Depending on the measurements, the machining of the base plate and of the keyhole slots are carried out in order to obtain, when cold, the desired clearances between the holding and supporting pieces.
After installing the baseplate and the keyhole slots, it is necessary to present the lower internals once again in order to check the clearances by gaining access via the manhole.
The operations necessary for installing the lower internals and their holding and supporting device are therefore lengthy and complicated and require the presence of a manhole in the central part of the support bottom.